firing into the ground can ricochet or create a spray of spall depending on what it hits. not good when its near you.
when firing into the air, when it comes down its just at normal falling speed as its expended all its energy on the way up. wont hurt anymore than a pebble of equivalent weight falling on you. also the direction i fire in is literally square miles of woods. no houses at all. so i fire maybe 10 degree from vertical in that direction.
A 'pebble' falling from several kilometers (without any acceleration except for gravity) will kill you I think. At least that's what I remember my calculation of over a decade ago. I've seen news articles too of (unintended) casualties from kalashnikovs being fired straight into the air in celebration in the middle east.
air resistance will be the limiting factor. i am unsure of the actual speed, depends on the bullet weight and size. i do know people who have been hit by falling birdshot (myself included, i do a lot of skeet shooting) and its nothing. but those pellets are smaller.
those kalashnikovs fire a heavy bullet. but were they indeed fire straight up? ive seen most fired maybe 30-40 degrees from vertical but thats just what i see on the news. not doubting you but all it takes is a slightly more horizontal trajectory and it will still have a lot of energy when the bullet hits the ground, as it wouldnt of expended its initial energy gaining altitude.